A bicycle gearshift comprises 1) a chain and toothed wheel system that transmits motion from the axle of the pedal cranks to a bicycle driving-wheel; and 2) means for selecting the toothed wheels with which the chain is engaged to change the motion transmission ratio.
A rear gearshift group is usually associated with the rear wheel hub of the bicycle. The group comprises at least two toothed wheels (also known as sprockets), a guide element for the chain (specifically, a rear derailleur or simply gearshift), and a control mechanism that displaces the guide element for the chain in the axial direction of the gearshift group so that the chain engages a predetermined toothed wheel in response to a manual manipulation of levers attached to the bicycle handlebars. A displacement from a toothed wheel with a smaller diameter to a toothed wheel with a larger diameter is indicated as “upward gear-shifting,” whereas a displacement from a toothed wheel with a larger diameter to a toothed wheel with a smaller diameter is indicated as “downward gear-shifting.” These terms are not the same and should not be confused with “up-shifting” and “down-shifting.”
Similarly, a front gearshift group is usually associated with the axle of the pedal cranks of the bicycle, and comprises 1) at least two toothed wheels also known as crowns or gears; 2) a guide element for the chain also known as front derailleur or derailleur, and 3) an actuator to displace the guide element for the chain.
Either the front gearshift group or the rear gearshift group can be replaced by a single toothed wheel, although this is less common in the rear gearshift group.
The mechanical gearshift's command mechanism comprises a steel cable slidably extending in a sheath (“Bowden cable”) between a manually actuated lever and the guide element for the chain. The actuation of the lever in a first direction exerts a traction on the guide element for the chain through the steel cable, whereas the actuation of the lever in a second, opposite direction exerts a thrust on the guide element for the chain through the steel cable, or rather it leaves the cable and the guide element for the chain free to be drawn by a return spring. The size of the displacement of the steel cable is set so that the stroke of the guide element for the chain is essentially equal to the distance or pitch between two adjacent toothed wheels of the gearshift group.
In an electronically servo-assisted gearshift, the command mechanism of the guide element for the chain comprises an actuator generally having an electric motor, and a lever system. An electronic control unit is also provided. The electronic control unit drives the actuators in order to carry out the gear-shiftings in a manual, automatic or semi-automatic mode. In the manually operated travel mode, the transmission ratio is input to the electronic control unit by the user through manual input means. In the automatically operated travel mode, the electronic control unit establishes the transmission ratio based upon an evaluation logic of the travel conditions. In the semi-automatically operated travel mode, the operation is a compromise between the manual and fully automatic.
The electronic control unit controls the actuators using logic positions (“logic values”) representing the physical positions of the various toothed wheels. These logic values are stored in suitable memory means.
In the rear and/or front gearshift group there can also be a transducer to detect the position of the actuator (and therefore of the guide element for the chain) and transmit the position to the electronic control unit.
Electronically servo-assisted bicycle gearshifts of the aforementioned type are described, for example, in U.S. Pat. Nos. 5,480,356, 5,470,277 and 5,865,454, and in European Patent Application EP 1 103 456, all assigned to or in the name of Campagnolo S.r.l.; and in U.S. Pat. No. 6,047,230 to Spencer et al. and German Patent Application No. DE 39 38 454 A1 to Ellsässer.
EP 1 103 456 describes a gearshift in which absolute type position transducers provide an electric signal indicating the absolute position of the derailleurs, so that after being switched (back) on, such transducers take into account the actual position of the derailleurs. This position could have slightly moved when the transducer was switched off because of vibrations caused during transportation.
Sometimes a rider needs to make a multiple gear-shifting, that is, move the chain from the toothed wheel (of the rear or front gearshift group) with which it is engaged—or current toothed wheel—to a toothed wheel not immediately adjacent to it. If this multiple gear-shifting moves the chain to a toothed wheel with a larger diameter, the process is called multiple upward gear-shifting; if it moves the chain to a toothed wheel with a smaller diameter, it is called multiple downward gear-shifting. Multiple gear-shifting using a mechanical command gearshift requires a single actuation of the lever (often for a longer time than for a single gear-shifting), or many actuations in quick succession. In either case, the actuation(s) impose(s) a displacement of the Bowden cable that moves the guide element for the chain equal to twice or three times the pitch between two adjacent toothed wheels (and thus moves the chain). In practice, displacing the chain guide element over greater than two or three times the pitch between two adjacent toothed wheels is difficult. Thus, the multiple gear-shifting is limited to a double or triple gear-shifting.
A solution that might be suitable for electronically commanded gearshifts is using an electronic control unit that, to carry out multiple gear-shifting, drives the actuator in a single step based upon the logic position representing the physical position of the target toothed wheel.
FIGS. 3 and 4 show the prior art for this. FIG. 3 shows a perspective view of the prior art where the main elements of the motion transmission system are shown. These are the toothed wheels of the rear gearshift group, indicated by reference numerals 11a-11i, the rear guide element or gearshift 14, the toothed wheels of the front gearshift group, of which only the outermost toothed wheel with the largest diameter is visible, the front guide element or derailleur 15, and the chain 13. The position of the gearshift 14 and of the chain 13 at the moment of a multiple gear-shifting from the toothed wheel 11c to the toothed wheel 11f is illustrated (i.e. a triple upwards gear-shifting, in this example), as carried out according to the prior art.
FIG. 4 schematically illustrates a view from the back side of the bicycle 1. Therefore, only the toothed wheels 11a-11i of the rear gearshift group 9, the rear guide element or gearshift 14 and the chain 13 can be seen. More specifically, the initial positions (to the right) and the end positions (to the left) of the gearshift 14 and of the chain 13, as well as the intermediate position of the chain 13 at the moment of a multiple gear-shifting from the toothed wheel 11c to the toothed wheel 11f carried out according to the prior art are illustrated.
The chain 13, drawn to move in the way X of the direction of axis A, is arranged at a large angle ALFA (formed between the plane of extension of the chain 13 and the planes of the toothed wheels 11), and is askew with respect to the intermediate toothed wheels 11d, 11e. This severe angle can cause the chain 13 to lose traction and/or get caught in the intermediate toothed wheels 11d, 11e, with the result that the rider can lose balance and fall. Moreover, the gearshift can be subject to mechanical damage. These problems are exacerbated during upward multiple gear-shifting.